Last update: Dec 26 2016
I replaced the hot water heater core with electric resistance elements taken from two 1500W Holmes HCH4062B-UM heaters. The elements have two stages of heating and are wired in parallel. The low stage with both elements makes too much heat; about 1600W at 10A. I should have used only one element instead of two. The second stage is disconnected.
Removing the heater core took 5 hours with two people. The service manual neglects to mention a few items in the removal procedure.
Fabricating the electric heater core took even longer than removing the hot water core. I made the frame from 26 gauge sheet metal using a simple sheet metal bender and pop rivets. If I were to do it again, I would instead use a high-temperature plastic such as McMaster Carr High-Temperature Chemical-Resistant CPVC which is rated to 200°F/93°C and Loctite Epoxy Plastic Bonder 1363118 which is rated to 180°F/82°C.
The back side of the electric heater core.
The wires pass through the firewall where the water pipes were.
Reinstalling the dash board support beam was a problem. The service manual does not specify torque values for the fasteners. Consequently, I over tightened a nut and broke a stud on the driver's side. To repair it, I drilled and tapped a hole through the broken stud for the next size smaller bolt.
The existing temperature control dial is repurposed to control a PWM circuit that adjusts the power to the electric heater core. The dial is connected to a linear potentiometer and a limit switch. The potentiometer tells the circuit the position of the dial. The limit switch cuts power to the circuit when the dial is turned to the coldest position.
The PWM cicuit controls a solid state relay as well as a a Gigavac P105 contactor, both located in the fuse and relay box. Power for the PWM circuit comes from the heater fan blower motor so that the heater can only be energized if the fan motor is also energized. The duty cycle ranges from a minimum of 25% to a maximum of 75%. At 50%, the temperature at the vent is about 130°F/54°C. An additional LED added to the front panel indicates the power level.
Circuit diagram below. Source code is here.